1. Field of the Invention
The present invention relates to a tensioner and more particularly to a tensioner for controlling the tension of a timing belt of an engine.
2. Description of the Related Art
A conventional tensioner 100 is shown in FIG. 8. There, the body 101 has a columnar hole 102 which has only one open end 103. In the lower portion of the columnar hole 102 is slidably inserted a plunger 104 which has a chamber 105 therein. A snap-ring 115 limits the upward movement thereof. A slight annular clearance 116 is formed between an inner surface of the columnar hole 102 and an outer surface of the plunger 104.
A passage 106 is formed in the plunger 104, and connects between the chamber 105 and a chamber 107 which is formed above the plunger 104 in the columnar hole 102.
At one end of the passage 106 is located a check-valve 108. The check-valve comprises a ball 109, a spring 110 and a retainer 111. The ball 109 is pressed onto one end of the passage 106 by the spring 110, and may be separated therefrom by the pressure in the passage 106. The retainer 111 holds the ball 109 and the spring 110 onto the plunger 104.
A spring 112 is interposed between a closed end 113 of the columnar hole and the plunger 104. The retainer 111 and a retainer 114 are located at opposite ends of the spring 112.
A rod 117 is inserted in the columnar hole 102, and is held by fixed rod-guides 118, 119. One end of the rod 117 contacts the upper surface of the plunger 104, and the other end thereof extends from the open end 103. A snap ring 120 is located in the lower portion of the rod 117, and limits the upward movement of the rod 117.
A piston 121 is located between the rod-guide 118 and the rod-guide 119 in the columnar hole 102. A seal-ring 122 is held between the inner surface of the columnar hole 102 and an outer surface of the piston 121. A seal member 123 is mounted between an inner surface of the piston 121 and an outer surface of the rod 117. A spring 124 is interposed between the rod-guide 119 and the piston 121. A dust-seal member 125 is located between the inner surface of the columnar hole 102 and the outer surface of the rod 117 near the open end 103.
Chambers 105, 107 are filled with a viscous fluid such as a damping oil.
In the above-mentioned conventional tensioner 100, an upper surface of the rod 117 receives the tension of a timing belt of an engine (not shown).
When the engine is at a low temperature, the tension of the timing belt is low, so that the rod 117 and the plunger 104 are urged upwardly by the spring 112 in the tensioner 100.
On the other hand, when the engine is at a high temperature, the tension of the timing belt is high, so that the rod 177 and the plunger 104 are urged downwardly against the spring 112. Here, the damping oil in the chamber 105 moves to the chamber 107 via the slight clearance 116.
The expansion of the damping oil by the high engine temperature is absorbed by the piston 121 moving upwardly against the urging force of the spring 124.
Here, the rod 117 receives both thrust and radial forces from the timing belt. A lower surface of the rod 117 is in planar surface contact with an upper surface of the plunger 104, so that the both forces are transmitted to the plunger 104. The radial force received by the plunger 104 causes rattling thereof, so that abrasion between the inner surface of the columnar hole 102 and the outer surface of the plunger 104 increases. Consequently, the value of the slight clearance 116 is increased, and the action of the plunger 104 is negatively influenced.
In addition, the snap ring 120 is needed for preventing the rod 117 from leaving the body 101. However, existence of the snap ring 120 makes manufacturing the tensioner 100 complicated and troublesome.